High performance analog-to-digital (A/D) converters are now widely used in modern communication systems. Many wireless receivers which handle a substantial dynamic range of signal amplitudes of a high data rate signal require the A/D converter to be operating at peak performance. One particular problem in such high performance A/D converters is differential non-linearity (DNL) errors, commonly referred to as spurs. DNL error is generally defined as the difference between an actual step width of an A/D converter and the ideal value of 1 least significant bit.
High performance A/D converters use multi-stage conversion. Although DNL errors occur in any A/D converter following a given probability density function (PDF), the effect is multiplied in multi-stage converters because the DNL pattern is repeated many times over. As such, DNL errors can dramatically impact the performance of a multi-stage A/D conversion process. Preventing the generation of DNL errors in the A/D conversion process, therefore, becomes paramount. In particular, one of the most important figures of merit for a digital receiver is spurious free dynamic range (SFDR). A receiver with excellent SFDR is able to receive at maximum data rate even when receive signals are at the low amplitude range of the receiver. DNL errors effectively decrease a receiver's SFDR rating.
A well-known technique called dithering is often required to maximize spurious free dynamic range. Dithering is the process of adding a non-correlated signal, such as pseudo random or broadband noise, to a desired analog signal prior to the analog input gate of the A/D converter. Although the injected dither does not eliminate the errors, it randomizes the DNL errors over the entire digitization process, thereby eliminating the concentration of DNL errors at a small number of codes. As a result, spurs are reduced with a negligible increase in the noise floor.
A common technique for generating the non-correlated signal is to use a thermal noise source which is uncorrelated to everything in the application universe. Adding noise that is not co-spectral with the desired signal is important, as otherwise, the benefits of the dithering are at least partially lost. In order to ensure this for most applications, the noise that is added can be low-pass filtered so as not to encroach on the frequency band of the desired signal.
Another issue that arises when combining the non-correlated noise signal with the desired signal is related to input loss caused by the circuit which adds the noise to it. In particular, the amount of power that the non-correlated signal requires (as typically supplied by a noise source) must be high enough to achieve the desired dithering effect. Conventionally, a directional coupler is utilized to add the non-correlated signal to the desired signal.
The directional coupler allows the desired signal to be combined with the non-correlated noise signal with a small loss in the signal path, and a substantial loss in the noise path. A 20 dB coupler, for instance, will result in a signal loss of 0.5 dB but a noise path loss of 20 dB. The noise source circuit must therefore overcome this loss by supplying an additional 20 dB of amplification. This additional 20 dB is not a trivial amount of power, and serves no useful purpose other than dissipating across the noise path of the directional coupler.
What is needed, therefore, are dithering techniques that efficiently combine a non-correlated noise signal with the desired signal to achieve high performance A/D conversion. In a more general sense, there is a need for a self-contained dithering module that generates the desired amplitude of noise as well as the appropriate noise signal roll-off for optimal results.